The present invention relates to a rolling bearing used under a high temperature circumstance and, in particular, it relates to a surface-treated rolling bearing in a working circumstance where water tends to intrude in lubricants, for example, a bearing for use in transmissions, hub units and engine auxiliaries of automobiles, a bearing for use in guide rolls or backup rolls in iron manufacturing machines and a bearing for use in dryer rolls in paper making machines.
For instance, in bearings for use in automobile wheel, water tends to intrude into lubricants in the inside of the bearings under the effect of muddy water on road or rain water. Further, also in a case of bearings for use in guide rolls in continuous casting facilities or for use in backup rolls in rolling mills for iron and steel materials, cooling water or water for rolling tends to intrude in lubricants. Further, in bearings for use in dryer rolls for paper making machines, it is said that steams tend to intrude to the inside of the bearings since they are used in the process of drying moistened paper.
In the rolling bearings, it has been generally known that durability of lubricants is greatly deteriorated when water intrudes into them. For example, it has been reported that the rolling life of a bearing is deteriorated about from one/severals to one/twenties when 6% water content intrudes into the lubricant compared with that before intrusion of water (xe2x80x9cThe Surface-initiated and Subsurface-initiated Rolling Contact Fatiguexe2x80x9d, NSK Bearing Journal No. 636, pp. 1-10, 1977, Furumura, Shirota, Hirakawa).
Further, it has been reported that when water intrudes into the inside of a bearing, for example, even when a slight amount of water content intrudes such as by about 100 ppm into the lubricants, the rolling fatigue strength of the bearing material is lowered as much as by 32 to 48% (xe2x80x9cEffects of Water and Oxygen During Rolling Contact Lubricationxe2x80x9d, Wear 12, pp 33-342, 1968, P. Schatzberg, I. M. Felsen).
Further as an example of J. A. Cirula, it has been reported for a test of intruding water in lubricants in a 4-ball rolling test that the bearing life was deteriorated to about {fraction (1/10)} compared with that before intrusion. In a rolling contact fatigue test for a bearing using steel balls charged with hydrogen, it has been shown that stainless steel balls have longer life than SUJ2 steel balls of second class bearing steel, but use of more expensive stainless steel balls than SUJ2 as a countermeasure for improving the bearings is practically difficult.
Then, in view of undesired effects of the water content on the bearing life, bearings have been sealed, for example, with contact rubber seals in order to prevent intrusion of water to the inside of rolling bearings used under a condition where water tends to intrude.
On the other hand, in bearings used under high rotational speed and high load such as bearings for use in automobile alternators, it has been reported a phenomenon that hydrogen is formed by decomposition of lubricants and the thus formed hydrogen atoms intrude into steels to cause peeling in an early stage as the phenomenon accompanied by the progress of remarkable increase in the speed of the equipments in recent years. For the early stage peeling phenomenon, it has been considered that water contained in lubricants (for example, water is contained always by about 0.1% in grease, or traction oils or ATF also contains water) is decomposed under high vibrations and high load, and hydrogen ions formed are adsorbed on the raceway surface and accumulated as hydrogen atoms to highly strained sites (in the vicinity of maximum shearing stress position) to cause stress corrosion cracking type peeling.
As the prior art intending for improving the working life of bearings used under high vibrations and high load accompanying such early peeling phenomenon, Japanese Patent Examined Publication No. 6-89783, for instance, discloses applying a heat treatment to bearing rings of grease-lubricated bearings in atmospheric air at a temperature of 200xc2x0 C. or lower, to form an oxide layer (black coating) of 0.1 to 2.5 xcexcm thickness on the rolling surfaces of the bearing rings to suppress intrusion of hydrogen to the bearing raceway surfaces caused by the decomposition of the lubricants.
Further, Japanese Utility Model Unexamined Publication No. 6-43349 also discloses a bearing in which an oxide layer of 0.1 to 2.5 xcexcm thickness is formed to the rolling surface of the roll bearing lubricated with lubricants. Further, Japanese Patent Unexamined Publication No. 5-26244 discloses a bearing in which at least a fixed bearing ring is formed of 1.5 to 6.0% Cr steel in a grease-sealed bearing, so that brittle peeling can be prevented by passivated layers of the material per se of the bearing ring.
Further, xe2x80x9cSAE Technical Paper: SAE 950944 (held on Feb. 27-Mar. 2, in 1995xe2x80x9d) discloses, in 1st to 14th paragraphs, a technique of preventing early peeling by analyzing fatigue mechanism of bearings for use in alternaters, replacing the sealed grease from E grease to M grease having a high damper effect, absorbing high vibrations and high load by the M grease while maintaining sufficient lubricant layers. For the early peeling phenomenon in bearings for use in engine auxiliaries, it is considered that if, for example, a grease with low damper effect is sealed in a bearing in a case where high vibrations/high loads exert by way of a belt from an engine crank to the bearing, lubricant layers are destroyed between rolling elements and raceway surfaces, hydrogen ions formed upon decomposition of water contained in the lubricants intrude through the fractured portions of the oil membranes and are absorbed to the raceway surfaces, to result in stress corrosion cracking type peeling.
A source of generating water in the bearings for use in engine auxiliaries is not restricted only to the water content in the grease. Since the bearings for use in the engine auxiliaries are often used in a high temperature state and they are cooled to an atmospheric temperature after stopping of the operation, it may be considered that water is formed by condensation from air present in slight spaces inside of the bearings or by intrusion of muddy water or the like from the outside.
In the sealing technique by using seals or the like as described above, it may be considered that the water content in the lubricants can be suppressed to about 10% or lower by the combined use of contact rubber seals attached to a chuck at the outside of the bearings and contact rubber seals incorporated in the bearing. However, there is a problem that intrusion of water into the lubricants can not be prevented completely. Even if intrusion of water from the outside can be prevented completely, it can not prevent water caused by condensation from air, for example, in the bearings for use in the engine auxiliaries which are stopped and cooled after operation at a high temperature.
On the other hand, the bearings disclosed in Japanese Patent Examined Publication No. 6-89783 and Japanese Utility Model Unexamined Publication No. 6-43349 intend to prevent intrusion of water into the inside of the bearings not by the rubber seal or the like but by directly suppressing and preventing intrusion of hydrogen caused by the decomposition of the lubricants to the raceway surfaces of bearings. However, also this technique involves a problems in view of facilities or treating time since it is necessary to apply a laborious treatment of dipping a bearing ring in an aqueous solution of sodium hydroxide heated to a low temperature to form tri-iron tetroxide layer (generally referred to as black coating) and further it includes a treatment of etching the rolling surface in an oxidative aqueous solution such as nitric acid, hydrochloric acid or sulfuric acid as other solvent to such an extent as causing coloration. Further, since the thickness of the oxide layer is as large as 2.5 xcexcm at the maximum, the bearing roughness is worsened when a large bearing load exerts, to possibly result in a problem of increase of vibrations.
On the other hand, it may be considered to heat the bearing material at high temperature in air as a means for forming the oxide layer to the bearing but, when the bearing is merely heated in air with no temperature control, oxidation occurs to form scales (black skins) of about several xcexcm thickness on the surface of the material. Unevenness of the scales may lead to loss of metals and possibly form initiation points for pit or the like. Further, if the bearing is merely left in atmospheric air, it may be considered that moisture in air and steels may possibly take place reaction to cause atmospheric corrosion.
Further, for the bearing disclosed in Japanese Patent Unexamined Publication No. 5-26244, it is described that a chromium oxide layer is formed on the surface of the bearing ring to inactivate the rolling surface by the use of 1.5-6% Cr steels at least to the fixed ring to prevent intrusion of hydrogen caused by decomposition of grease. However, as shown in xe2x80x9cPretext for Japan Tribology Conference (Tokyo, 1995-5) pp 551-554xe2x80x9d, since auto-slipping is caused at the inlet of the fixed rings in bearings for use in engine auxiliaries used under high vibrations/high load, the oxide layer intended for providing the damper effect is cut and, as a result, the outer ring in which early peeling occurs frequently undergoes load directly and it is actually difficult to prevent early peeling in the fixed ring.
The present invention has been accomplished for overcoming such disadvantages in the prior art and it is an object thereof to provide a surface-treated rolling bearing by forming an iron/chromium oxide series oxide layer of about 1 to 1000 nm thickness on the surface of bearing members used in a circumstance in which water is incorporated in lubricants, thereby enabling to suppress formation of hydrogen caused by decomposition of incorporated water and intrusion of hydrogen atoms into the matrix of the members and remarkably improve the bearing life, as well as a manufacturing method thereof.
For attaining such an object, the invention described in claims 1 to 7 of the present invention relates to a surface-treated rolling bearing.
The surface-treated rolling bearing according to the present invention is a rolling bearing in which a plurality of rolling elements are arranged between bearing rings comprising a fixed ring and a rotational ring in use, wherein an oxide layer of 1 to 1000 nm thickness comprising an iron/chromium oxide series (hereinafter also referred to as a re-heating oxide layer) is provided at least to one of the raceway surfaces of the bearing rings or the rolling surfaces of rolling elements. This prevents intrusion of water incorporated in lubricants and formation of hydrogen caused by decomposition of incorporated water and intrusion of hydrogen atoms into the matrix of bearing members thereby preventing early peeling at the raceway surfaces of the bearing rings or the raceway surfaces of the rolling elements to greatly improve the rolling contact life.
In this case, if the thickness of the oxide layer is less than 1 nm, it is difficult to inhibit intrusion of the hydrogen atoms into the matrix of the bearing members and early peeling on the raceway surfaces of the bearing rings or the rolling surfaces of the rolling elements can not be prevented completely. A preferred thickness of the oxide layer is from 5 to 1000 nm. This can prevent intrusion of the hydrogen atoms into the matrix of the bearing members more reliably.
Further, the surface hardness of the raceway surface of the bearing ring or the rolling contact surface of the rolling elements formed with the oxide layer is Hv 650 or more.
Further, when the amount of residual austenite in at least one of the bearing ring or the rolling element is 6.0 vol % or less, dimensional expansion caused by decomposition of the residual austenite at high temperature is suppressed to improve the bearing function in view of creep prevention. A more preferred result is obtainable when the amount of residual austenite is 2.0 vol % or less.
The ingredient composition of the alloy steel for the bearing member in the present invention varies depending on whether carburization or carbo-nitridation is applied or not to the bearing members. In a case of so-called direct hardening not applying carburization or carbo-nitridation, the ingredient composition of the alloy steel for at least one of the bearing ring or the rolling element to be formed with the oxide layer comprises C: 0.7-0.93% by weight, Si: 0.15-0.50% by weight, Mn: 0.50-1.10% by weight and Cr: 0.30-0.65% by weight.
On the other hand, when carburization or carbo-nitridation is applied, the ingredient composition of the alloy steel for at least one of the bearing ring or the rolling element to be formed with the oxide layer comprises C: 0.2-0.6% by weight, Si: 0.7-1.5% by weight, Cr: 0.5-2.0% by weight and Mo: 0.5-2.0% by weight, to which carburization or carbo-nitridation is applied. After applying carburization or carbo-nitridation and further applying grinding treatment, it is desirable that the surface C concentration is 0.8-1.2% by weight or the surface N concentration is 0.05-0.5% by weight.
The invention described relates to a method of manufacturing a surface-treated rolling bearing.
A first method of manufacturing a surface-treated rolling bearing according to the present invention comprises a step of applying a hardening treatment to at least one member of bearing rings or rolling elements and then applying a tempering treatment within a temperature range from 240 to 320xc2x0 C., and a step of applying re-heating to the tempered member in air at a temperature lower by 30xc2x0 C. or more than the tempering temperature thereby forming an oxide layer of 1 to 1000 nm thickness comprising iron/chromium oxide series to the surface of the member to increase the surface hardness to Hv 650 or higher.
Further a second manufacturing method comprises a step of applying a hardening treatment to at least one member of bearing rings or rolling elements and then applying a tempering treatment within a temperature range from 400 to 500xc2x0 C., and a step of applying re-heating to the tempered member in air at a temperature of lower than 400xc2x0 C. (preferably 150-350xc2x0 C.), thereby forming an oxide layer of 1 to 1000 nm thickness comprising iron/chromium oxide series to the surface of the member to increase the surface hardness to Hv 650 or higher (preferably Hv 700 or higher). In this second manufacturing method, the tempering temperature is higher than that in the first manufacturing method and the temperature of applying re-heating in air to form the oxide layer comprising iron/chromium series is also higher than that in the former, and this is suitable to a case in which Mo having high tempering resistance is contained in the ingredient composition for the member to be heat treated.